Condition control system



-' June 2, 1942. I c. L l- ALL ETAL 2,285,204

CONDITION con'mofl SYSTEM.

Fiied April 1, 1941 3 Sheets=$heet 2 Fig. 6. a1 86- T T 510? Y E p- STOP PERCENT OUTPUT TEMPERATURE MODULATING STOP 49 PERCENT TRAVEL m E: g O

TBTWERATURE MODULATlNG g ENVELOPE TI ME Inventors:

Chester I. Ha; "Fredrick S. Marcellys,

NW 6. A

Their Attorney.

June 2, 94 Q1. HALumL 2,285,204

CONDITION CONTROL SYSTEM FiledwApril l. 1941 3 Sheets-Shea}. 5

. 7, INCREASE TEMP.

. V Y V V V 64 DECREASE TC/1R F g- 8- J I I 5 DECREASE J TEMR Inventors:

' ChesterLHall, Fregirick S. Marcellus Them Attorney.

Patented alune 2, 1942 CONDITION CONTROL SYSTEM Chester I. Hail, Schenectady, and Fredrick Marcellus, Scotia, N. Y., asslgnors General Electric Company, a corporation cl New York Application April 1, 1941, Serial. No. 386,314

" 23 Claims.

values in response to whether the temperature of the space is above or below the desired value. For example, in an oil burning steam heating system the output of the heater is varied almost instantaneously from zero to substantially its maximum value, the time of!" and the time on being directly dependent upon the position of a room thermostat. In the on-oil type of control it is not necessary that the output be varied from zero to one hundred percent, but it may be varied between any two desired values. p

The second fundamental type o1 control is known as floating control in which the output of the heater or other condition changing means or the input to the conditioned space is varied relatively slowly between predetermined limits as distinguished from substantially instantaneously the on-oil? control. For example, a valve controlling the flow of steam through aconduit may he moved slowly from full open to full closed by reversible motor responsive directly to the position of a room thermostat.

The third iundamental type of control is lrnown as proportional" control, and is that type in which the output of a heater or other temperature control systems has particular applicati n perature of a magnitude greater than that permlssihle closely regulated processes. Our in ven'tion provides means may he rolled to the control of on-chi system temperature fluctuations by overshooting of the control point are reduced to analmost imperceptlble'value and whereby the system is endowed with a proportional characteristic.

The simple floating control mentioned above inherently produces less violent fluctuation in the temperature than on-oil. control because of the slow movement of the condition controlling element. This type of control however is open to the objection that it is very slow to respond to changes in the condition. Because of the fact that in a floating control the condition controlling element is moved at a constant rate, the slow response of a floating control is particularly objectionable where relatively large deviations of a condition from normal are to be compensated. Accordingly, our invention, when applied to a floating type or control, provides an improved system in which the rate of movement of the condition controlling member is made proportional to the magnitude of the deviation of the condition being controlled from the desired value.

' With such an arrangement our control system no condition changing means or the input to the conditioned space is varied in direct proportion to the position of a condition responsive memher. For example, a valve controlling the flow of steam through a conduit may be directly connected to a thermostat, whereby the valve takes up a definite position for each position of the.

thermostat.

While the on-oil type of control is characterized by relatively quick response, it is often found objectionable in heating systems using a large input of energy, such as industrial electric heating furnaces, because of'the tendency of the relatively high rate of heat input to cause overtemperature during the on period and conversely to cause'under-temperature during the ofi,perlod. Such overshooting of the control vide means for controlling the temperature of an I enclosure within very close limits.

It is a still further object of our invention to provide a control system for an industrial heating furnace which is capable of maintaining a selected temperature'wlth only very slight and substantially imperceptible fluctuations.

It is a still further object of our invention to provide" means for modifying an on-ofi type. of

tion from its normal value.

Still another object'of our invention is to provide a new and improved type of continuously oscillating operator for cycllcly varying the output of a heater or other condition changing means or the input to a conditioned space.

It is a still further object of our invention to "provide a continuously oscillating rotatable operpoint frequently causes a cyclic variation 0! tern- 0 9. 901 having condition increasing and condition to provide decreasing positions with means for varying the speed of the operator in accordance with the extent of deviation of the condition from its normal value.

It is a specific object of our invention to provide a variable speed oscillating operator for regulating an on-off control system adapted to maintain nearly constant the temperature of an electric furnace.

In accordance with our invention we utilize an induction disk relay provided with a contact mechanism which has spaced on and "off positions and with windings specially arranged to produce automatic reversal of the direction of rotation of the relay between its positions, the rotations in opposite directions taking place at speeds determined by the extent of deviation of the condition from its normal value. Specifically,

the windings are arranged to produceupon the tion .being regulated. Since the difference between the torques determines the speed of rotation of the induction disk in each direction, the speeds of rotation in opposite directions are varied inversely in accordance with the magnitude of the deviation of the condition from its normal value, whereby the ratio of time off to time on per cycle is varied to correspondingly vary the average output per cycle of the heating means.

For a more complete understanding of our invention, and for a further appreciation of its many objects and advantages, reference should now be had to the following detailed specification taken in connection with the accompanying drawings .in which Fig. 1 is an exploded perspective view of our induction relay; Fig. 2 is a detailed view partly in section showing the driving connection between the induction disk and the contact mechanism of our induction relay; Fig.

.3 is a detailed view partly in section of one suitwith coil connections 13 and terminal connections ll. The relay core structure I2. is multipolar and comprises a central pole l5 carrying an exciting coil l6 and two radially displaced poles l1 and I8 carrying energizing coils I 9 and 20 respectively. The energizing coil I9 comprises two windings connected in opposing relationship, one winding being arranged to set up a flux of the order of two per cent of the flux of the other winding, as will be more fully de-' operation with the opposing poles I5, I! and I3 of the core structure l2. The bearings 2! and 28 are mounted upon a face plate 29 which is adapted to be attached ,to the relay casing III by bolts 30 at opposite sides of the plate. The face plate 29 has formed on the front surface thereof two bosses 3| adapted to support a mounting plate 32. Rotatably mounted upon the plate 32 is a mercury button contact mechanism 33 vhaving its terminals connected to terminal blocks 34 and 35 by flexible electric conductors 36. The relay shaft 26 is connected to rotate the mercury button through a gear reduction sho in detail in Fig. 2. Referring to Fig. 2, a bearing plate schematic diagram of circuit connections show-v ing our induction relay applied to the control of an on-off reactor system-of electric furnace control; Fig. 6 is a time chart showing various conditions of operation of the system with a constant temperature setting; Fig. 7 is a time chart.

showing various conditions of operation of the system when the temperature setting is changed; Fig. .8 is a schematic diagram of circuit connections showing our invention applied to' the onoff" control of an oil burner motor for a domestic the structure of an induction relay forming part of our invention. The relay casing, indicated generally at I, is recessed at I I for the reception of a core and coil structure indicated generally by the reference numeral l2, and' is provided .of the mounting plate 32. Between the plates 32 and ill is mounted a shaft ll carrying at one end a beveled gear 42 for cooperation with a beveled gear 43 on theshaft 26. Near the other end of the shaft ll is mounted a'spur gear 44 which meshes with a spur gear 146 on a shaft 46. The mercury button 33 is mounted in a hub 41 attached to the shaft 46 and carryinga radially projecting arm 43. Preferably the hub 41 is formed of insulating material. The radially projecting arm 43 is adapted to engage-a pair of stops 36 and 60 set in the mounting plate 32 for the purpose of limiting rotation of the mercury button 33.

' Referring now to Fig. 3, the mercury button contact mechanism which we. prefer to use as a contact mechanism is shown in detail. Such a rotatable mercury button is described and claimed in Patents 2,101,092 and 2,101,093 to J. H. Payne, issuedDecember 7, 1937, and assigned to the same assignee as the present application. The mercury button has been shown partly in section in order to illustrate the spaced relation of the on" and "off" positions of the button. In Fig. 3v the reference numeral 55 indicates a pool of mercury and 66 represents an insulating baille apertured at 51. As is fully explained in the above mentioned patents to Payne, the circuit through the mercury button is completed when the mercury passes through the aperture 51 in the baffle 66. Because of the relatively great'surface tension of mercury it is found that the. aperture 61 must 'be almost completely submerged below the surface of the mercury before the mercury will flow through the aperture. This position of the mercury button is shown in full lines in Fig. 3. Also, because of the high surface tension of mercury, it is necessary almost completely to withdraw the aperture from the pool of mercury before the conducting connection through the aperture 61 is broken. This position of the mercury button is indicated by the dotted aperture 51 of Fig. 3. course, the angular distance between the full and dotted line positions shown in Fig. 3 will depend largely upon the diameter of the aperture 51. With the size aperture which we prefer to use, it has been found that the on" and oil' positions of the mercury button 33 are angularly spaced apart by approximately eighteen degrees. We wish to have it understood, of course. that, if desired, other contact mechanisms having spaced onand oil positions may be used. For example, a cam operated switch or spring hissed toggle switch may be suitably arranged for operation in corn nection with our invention.

Referring now to Figs. in how our ctio reacted to regulate the in controlled electric hea rig y he coua reactor The . nace contains a resistance heating element ti which isconnected for on-off control through a saturable core reactor 52. When the resistance heater ti is controlled through a reactor such as 62, the "on and off limiting values of the output or" the resistor 61 preferably lie inter mediate zero and one hundred per cent. As is well understood by those skilled in the art, this arrangement of itself tends to prevent tempera ture fluctuations due to overshooting of the con trol point. The heating resistance 6! is connected for energization from a pair of line alternating current conductors S3 and 64. A direct current winding 65 of the saturable core reactor 52 is energized through a rectifier bridge to which is connected to be controlled by the mercury button 33 or" an induction relay accordto our invention. In Fig. 5 the induction relay is represented generally hy the reference numeral til. Preferably the reactor $22 is constructed to have impedance such that when is unsaturated a current of substantial heating value is supplied to the resistance heater 55, such as approximately 65% of the maximum heating current when the reactor is fully saturated. as indicated in Figs. 5 and Z.

The exciting winding l6 of the relay 6? is connected for energization directly from the secondary terminals of a control transformer 58, the primary terminalsohwhich are connected between. the line conductors 53 and M. The potential exciting coil 16 is preferably highly inductive and therefore sets up a flux in the relay core structure which is ninety electrical degrees out of phase with the secondary voltage of the control transformer 68. This flux lags behind the transformer secondary voltage, and may be represented by the vector he of Fig. 4. For cooperation with the flux of the coil W to produce a torque in one direction upon the disk 25, an energizing winding 69 is arranged upon the leg of the relay. The winding 69 is themain winding of the coil i901 Fig. l. and is substantially non'inductive relative to the coil 45. For energization the windingtii is connected to the secondary winding of the control transformer 58 through a substantially nondnductive circuit in may be assumed for purposes of illustration that the combined efiect of the windings I8 and 89 tends to produce rotation of the disk 25 in a direction to rotate the mercury button 33 counter-clockwise toward its "on" position.

For producing a torque in the opposite direction upon the disk 25 the coil 20, similar to' the winding 59 but reversely wound. is positioned upon the log it oi the relay. The coil 2! is connected for energlzation seen a voltage divider "it or other suitable voltage varying device. The voltage divider it is connected for energiaation across the secondary terminals or the control transformer 68. The circuit of the coil 2 3 is primarily resistive and 1 ludes a temperature sensitive resistance va: g element The temperature sensitive element comprises a st-or having arlced n .tive temporal.

type described and.

plication 01' Chester Hall. Io. 3219M, April 1940, and assigned to the same as signee as the present application. as do" ri'oed in the cop-ending application the element l5 comprises an outer electrode in the form of a tube,

an inner electrode centrally located in the tube, and a sintered mass of granular resistance material within the tubular electrode embedding the lower end of the inner electrode. One such sintered granular resistance material may comprise magnesium oxide, sodium silicate and copper oxide. The flux set up by the coil 20 is substantial y one hundred and eighty electrical degrees out of phase with the voltage of the secondary winding the control transformer 88 due to the reversed winding of the coil 28. Thus it will be evident that the time phase of the flux set up by the coil 29 will d r from that of the crux set up y the winding 59 by approxione hund l eighty electrical degross and will differ from that of the coil 96 by approximately ninety electrical degrees in a lagglng sense. The flux of the coil 20 may be represented by the vector 20 of Fig. 4.

If desired the one hundred and eighty degree phase shift between the fluxes c509 and mo may he produced by winding the coil 20 in the same di. rection as the winding 59 and energizing the voltage divider H from the secondary windings of the control transformer 58 through an insulating transformer which may have a 1 to 1 ratio,

and will be more fully described with reference to Fig 3.

Each of the Fluxes also and em interacts with the constant exciting flux 16 to produce a torque upon the conducting disk 25. Each torque depends for its direction upon the direction of phase angular shift between the two fluxes producing it. and for its magn tude upon the magnitudes of the fluxes and the magnitude of the phase angle between them. For example, consider the varlbus torques established. Most of the flux set up in the central pole it by the coil it passes across the narrow air gaps above the disk 25 and returns without cutting the disk. Some of the exciiing flux. however. cuts the disk 25 and returns through the radially d splaced poles l1 and 18.

This portion of the exc t ng fiux sets up circulating currents in the conducting disk which lag behind the flux 016 by substantially ninety electrical'degrees. Since these circulating currents are in phase'with the flux can and one hundred and eighty degrees out of phase with the mo, their magnetic fields are acted upon by the fluxes an and zinc to produce oppositely directed torques. Similarly, the fluxes in the radially displaced ed torques.

dred and eighty degrees out of phase with the flux 4216. Therefore the flux 1o acts upon the magnetic fields of the circulating currents set up by the fluxes 4m and 9520 to produce oppositely directed torques. Inspection will show that the torque produced by the flux e9 acting upon the currents set up by the flux 1e is in the same direction as the torque produced by the flux 1u acting upon the currents set up by the flux' 69. The same is true of the fluxes due and 4:20.

In order to cause cyclic automatic reversal in the direction of rotation of the induction disk 25, we provide a small reversing winding 1'6 which may .be wound upon the same spool with the winding 69 and is connected to set up a flux opposing the flux set up by the winding 69. The winding 16 is primarily resistive and is connected for energization directly from thesecondary winding of the control transformer 68 in series with the contact mechanism 33. It will therefore be seen that the winding 16 will be enerazsaaoa on position thereversing winding 13 is energized during the on" periods of the. furnace and will be deenergized during the off period of furnace operation. The reversing winding 16 is designed to set up a flux having a. magnitude of the order of two per cent of the magnitude of the flux set up by the winding 39, and the energization of the winding 13 may be varied by changing temperature sensitive resistor 15 in circuit, with the coil 20, the magnitude of the flux 2o is dependent upon the temperature within'the furnace 60. The normal value of the flux dazo, that is the value of the flux 2o when" the furnace is at the desired temperature, is intermediate the value of the flux eo andthe resultant flux disk-#16. For purposes of illustration it may be assumed that the normal value of the variable ,flux M0 is less than the value of the fixed flux #10: by onehalf'the value of the fixed flux #176. As will be apparent from the following description, this means that as the electric furnace 6!! is cyclicly turned on and off by our induction relay an average furnace output midway between the limiting points of furnace output will be Just sufficient to balance the heat-losses.

Referring once more to Fig; 5, it will be seen that the direct current winding 65 of the saturable core reactor 62 and the reversing winding 16 of the relay 61 will be energized when the mercury button 33 is in its on position and will remain energized until the mercury button arrives at its oil? position. Likewise the direct current windingof the reactor and the reversing Winding of the relay will be deenergized at the time the mercury button arrives at its off position and will remain deenergized until the mercury button reaches its on position.

- From Fig. 4, it will now be apparent that the three magnetic fluxes om, sa and2o produces upon the rotatable disk 25 two oppositely direct- Normally, the torque set up y h fluxes $16 and sa, tending to rotate the mercury button 33 to its on. position, is predominant.

- 61 will oscillate back and forth cyclicly to turn the furnace 60 on and off. For purposes of illustration, it may be assumed that with a cycle time of fifty per cent on and fifty per cent off the average furnace output will just balance the heat losses.

Referring now to Figs. 6 and 7, we have shown a number of time charts by which the operation of our furnace control system may be illustrated. We wish to have it understood, of course, that these charts have been drawn to a disproportionate scale in order more clearly to illustrate our invention, and that therefore the magnitude of the temperature fluctuations shown thereon is not to be considered proportionate to that actually obtained in practice. Fig. 6 illustrates the operation of our system when the desired temperature setting is allowed to remain fixed at one selected value and under conditions of temporary load change on the furnace. Fig. '7 illustrates the operation of our invention when the temperature setting of the system is changed.

Referring first to Fig. 6, the curve A represents the temperature of the furnace ill, the curve B represents the percentage output of the heating resistor GI, and the curve C represents the angular travel of mercury button 33 in degrees; all of the curves being drawn with time as the abscissa. The curves D and E are heating and cooling curves respectively of the furnace 30. Beginning with a time To when the temperature of the furnace is at its desired value, it will be observed that the mercury switch cycles from on" to ofl continuously, completing a com-' plete cycle in a time To to T1. As indicated by the output curve, the furnace is on for fifty per cent of this cycle and off for fifty per cent. As previously stated this condition is merely assumed for the purpose of illustration, but it will be understood, of course, that any other condition of balance may exist in practice. As the system cycles from off" to on with the temperature remaining at the desired value, small fluctuations in temperature such as that indicated at uponthe curve A, will of course occur due to the continual turning of! and on of the furnace. It has been found however that these normal fluctuations are imperceptibly small. By way of example, in one sample installation upon which we have run tests over a considerable period of time, it has been found that a temperature of 700 degrees F. in an electric furnace may be maintained with a maximum deviation of less than one degree Fahrenheit.

- As a further illustration, however, let it be ascause, such as the opening of a furnace door.

Let it be further assumed that the decrease in temperature of the furnace at the time T2, while it reduces the flux Mo to some extent, is not sufflcient to reduce the flux Mo to a value less than that of ciao-duo. The reduction of the flux 4 results from the increased resistance of the rerelay was rotating the mercury switch in a countor-clockwise direction under the influence of the resultant flux 4 oe-2o. with the decrease in furnace temperature the flux 4 2o was reduced so that the'opposing torque exerted upon the disk 25 by the flux4m was reduced. Because of the reduced opposing torque the resultant torque due to the flux duo-4m is increased and the disk moves toward the on" position at a more-rapid rate of speed. as is shown by the increased slope of the curve C of Fig. 6 at the time T2. When the mercury switch arrives at'its on" position the furnace is turned on and the reversing winding I6 is energized. With the flux of the winding 16 opposing the flux of the'winding 89 the resulting flux oo-1s is reduced to a value less than the diminished value of due. However, the resultant flux 2o-eo+is is small compared to the resultant flux e92o. In other words, for operation of the mercury switch in the clockwise direction to turn the furnace off the operating flux duo is only slightly larger than the restraining flux asp-e1 while for operation of the switch in the counter-clockwise direction to turn the furnace on the operating flux 4m is considerably larger than the restraining flux e20. For this reason the relay rotates toward the on position at a much higher rate of speed than normal, as shown by the portion 8! of the curve C in Fig. 6, and rotates toward the ofi position ata much lower rate of speed than normal, as indicated by the slope of the portion 82 of the curve C. New length of'the cycle, however, remains substantially unchanged as indicated in the drawings. Thus it will be seen that during the cycle :hetween the times T2 and T3 the ratio of time on to time off and thus the average output per cycle of the furnace has been increased by inversely varying the speeds of rotation of the induction relay opposite directions of travel, whereby the average output per cycle of the furnace has been increasedv in proportion. to the deviation of the furnace temperature from normal. The effect of the relatively long on period oi the iurnace during the cycle T2 to T3 may be seen from the portion 33 of curve A of. ii. The portion of the curve ll. follows the heating curve D of the furnace. at the time T3 the deviation .7 teniperatlue from its normal value is is c it was shortly after the time There value ii @920, though somewha below its'norm clue, is greater than. 4 w

the beginning or? the previous cycle, will now be apparent that the speed of rotation oi the induction disk toward the on position, as shown by the slope of the portion $4 of the curve C of 5, somewhat greater than normal though somewhat less than its speed of rotation in this direction during the previous cycle. Likethe speed of rotation of the induction disk toward its off position as shown by the slope of the portion 85 of the curve C of Fig. 6 u illbe slightly greater than the of rotation inclicated by the slope of the portion 82 of the curve but will still be slightly less normal. There fore during the cycle inchlcated between the times To and It the ratio oi" time on to time cit and theaverage output per cycle or the furnace will still be above normal, but be somewhat remood with respect to the ratio of time onto time iii) off during the cycle '1': to T3. As indicated upon 18. 6, it is assumed that at the time T4 the furnace has again reached its normal temperature and that the cycling continues in a normal'manner until a tim T5.

At the time Ts it may be assumed that some external cause rwults in a sudden increase infurnace temperature-The sudden increase in furnace temperature at the time-T5 has been as sumed to be such that the resulting increase in the magnitude of the flux 4:20 is not suflicient to make on larger than 4m. Now by comparison with the operation of our system previously explained in detail it will be apparent that, since the furnace was of! at the time T5, the increase in the value 2o will so restrain the disk 25 that its rotation to the on position under the influence of ae-zo will proceed at a speed lower than normal as indicated by the portion II of the curve C. Similarly, rotation of the disk toward the off position under the influence of the increased resultant flux zoee+7s will proceed at a rate of speed' greater than normal as indicated by the slope of the portion 81 of the curve C. Thus, during the cycle following the time T5 the average furnace output per cycle is decreased by inversely changing the speeds of rotation of the induction disk in opposite directions in accordance with the value of the temperature responsive flux duo. As indicated on Fig. 6, the two cycles following the portion 81 of the curve C take place with the induction disk rotating at abnormal speeds determined .by the temperature of the furnace, and are such as to gradually increase the average output per cycle of the furnace to its normal value. The deviation' from average'fu'rnace output per cycle is proportional to the deviation of furnace temper" ature from normal.

Let it new be assumed that without changing the temperature setting of the furnace, the furuace temperature is suddenly decreased at a time To to e. value below the lower limit of the region marked modulating envelope" on 6. This means that the furnace temperature has been reduced. to a value suficiently low to diminish the magnitude of the flux $20 to less than that of sa--te. Referring now to curves E and ill of Fig. 5, it will be seen that the furnace had just been turned off at the time To. At this time, therefore. the mercury switch was being rotated toward its on position under the influence of the resultant our-om. Due to thedimlnution of the retarding flux ize the rotation of the mercury button toward its on position procoeds at a greater speed because of the increased value 01 the resultant flux pee-p20. The in creased rate of speed is indicated by the slope of the portion 88 of the curve C of Fig. 6. when the mercury switch arrives at its on position the furnace is t med on, as at a time T7 indicated on curve B, and the reversing winding it is connected to oppose the flux of the winding Ell. However, because of the large diminution of furnace temperature the iiux sa-'m is still larger than the 520. The mercury button therefore continues to rotate in theclockwise di reotion until the pin 68 engages the stop W, as indicated at 89 of curve C oi Fig. 6. The mer cur-y button will now remain against the stop 8 until a Ta at which the furnace tempera ture is increased to a point where @520 is equal to oatm. At this time the opposing fluxes operating upon the induction disk 25 are exactly balanced. Shortly after this time when the furnace temperature has further. increased to a point where the flux 2o is slightly larger than the flux 60-76 the mercury switch will begin to rotate in the clockwise direction at a gradually increasing rate in accordance with the gradually increasing magnitude of the flux can in response to increasing furnace temperature. As indicated by the slope of the portion 90 of the curve C of Fig. 6, however, the mercury button reaches its of! position at a time To before it attains its normal speed and before the temperature has returned to normal. At the time To therefore the furnace is turned off. Since the temperature and hence the flux 2o are still below normal, the speed of the mercury button back toward its on position under the influence of the resultant flux es-2o is greater than normal. as indicated by the slope of the portion 9] of the curve C. Similarly the slope of the portion 82 of thecurve C is slightly less than normal. The ratio of time on to time off during the cycle from To to Tm is thus slightly increased. Since the temperature of the furnace at the time T10 differs from its normal value by only the amount of its normal cyclic deviation, as shown by the curve A, the flux 4:20 and hence the speed of the induction disk will benormal; and thus the portion 83 of the curve C will have approximately its normal slope. The system therefore now begins to cycle in its normal manner as indicated during the three cycles following the time Tin. v

If it is now assumed that at a time T11 the temperature of the furnace is suddenly increased by some external cause to a value suflicient to increase the flux can to a value greater than the flux mathe mercury button will be rotated beyond it off'- position and into engagement with the fixed stop 50. By analogy to the regulating action described immediately above, it will be apparent that the average output per cycle of the furnace will be reduced in accordance with the magnitude of the deviation of furnace temperature from its normal value, as reflected in the relative speeds of rotation of the induction disk in opposite direction under the influence of the temperature responsive flux 2o.

Referring now to Fig. 7, we have shown a representative number of examples of the manner in which our system operates when the normal temperature setting of the apparatus is changed with the ambient temperature and other conditions of furnace operation remaining the same. Let it be assumed that the time To the apparatus is set to maintain the temperature of the furnace at 700 degrees F. and is cycling in the normal manner to maintain the temperature at this point with very slight fluctuations. As before, it will be assumed that-the ratio of time on to time off during each cycle is l to l tomaintain balanced conditions. If now at a time T1 the temperature setting of the apparatus is changed to maintain a normal temperature of, for example, 500 degrees F. the conditions of furnace operation are indicated at Fig. 7. The change in the setting of the apparatus is effected by changing the position of the sliding contact of the voltage divider H to impress a greater voltage across the circuit including the coil 20 and the temperature sensitive resistor 15 in series.- Since for operation of our system at the selected such a point that when the resistance of the resistor 15 has increased to its value at 500 degrees F. the current flowing through the coil will have approximately its normal value. Under the conditions assumed the value of the flux 20 must return 'to aproxlmately a9-l/216. The flux 4 2o will not return to exactly-the magnitude as will be explained hereinafter. The process by which the temperature of the furnace, and thus the resistance of the-resistor 15, is changed to reestablish 'the normal value of the current through the coil 20 is indicated upon the curve A1, B1 and C1 of Fig; '7, which correspond to the curves A, B and C of Fig. 6. The heating and cooling curves D and E of the furnace have also been indicated on Fig. 7. At the time T1 the mercury switch has just arrived at its off position. Had the temperature setting not been changed the mercury switch would at this time begin to rotate in counter-clockwise direction under the influence of the resultant flux s92o. However, due to the change in temperature setting, the value of the flux 620 has been increased to a value greatly in excess of the flux c569. Therefore, the mercury switch continues to rotate in the clockwise direction until the pin 48 engages the stop .50, thereby maintaining the furnace in its off position. As indicated by the portion I00 of the curve C1, the mercury switch will remain against the stop 50 until a time T2 when the furnace temperature comes just within the new modulating envelope determined by the new temperature setting. At this time the flux zo will be just equal to the flux mo and the rotatable disk 25 will be balanced. Since the mercury switch is still against its off" stop, however, the furnace remains off and the temperature continues to fall with a resulting continued decrease in the magnitude of the flux 2o. As the flux 2o decreases below the value of the flux sa in response to the decrease in temperature of the furnace, the mecury button 32 rotates in a counterclockwise direction toward its on" position at a gradually increasing rate. As shown by the portion IOI of the curve C1, the mercury button reaches its on position before its speed comes posite direction relatively Quickly as shown at I02 on the curve C1. The ratio of time on to time off per cycle is thus gradually brought back to a normal value over the course of a number of cycles until at a time T4, when the temperature has arrived at the new setting, the apparatus again begins to cycle in a regular manner.

' The normal cycle of the apparatus will now be because a smaller average furnace output per cycle is sufficient to maintain a temperature of 500 degrees. It will be observed from Figs. 6 and 7 that it has been assumed that the heating and cooling curves have approximately the same slope at a temperature of 700 degrees. Since the slope of these curves is different at 500 degrees,

by the voltage divider 1| must be increased to the furnace will heat up more quickly and will cool more slowly as indicated by the portions I02 and H13 respectively of the curve A1 of Fig. 7.

The difference in the heating and cooling rates immediately above.

2,285,204 while the total cycle time will remain approximately the same, the average input per cycle, or the ratio of time on to time ofi, wil1 be somewhat reduced in comparison to the normal cycle at 700 degrees. I

It now it is assumed that at a time T5 indicated on Fig. 'l, the temperature setting of the apparatus is changed to maintain a normal temperature of, for example, 900 degrees F. the mercury button will rotate in the counter-clockwise directlonto maintain the furnace on continuously until the new normal temperature is reached, and will thereupon reestablish normal cycling operation under conditions determined by the relative slopes of theheating and cooling curves at 900 degrees F.- It is believed that this .maintain a temperature of 900 degrees than is required to maintain 500 or 700 degrees. This is indicated by the relative slopes of the heating and cooling portions of the curve A1 after the tially one hundred and'eighty electrical degrees,

former 68 through an insulating transformer I06 preferably .having a 1 to 1 ratio. It will be understood .by those skilled in the art that it the insulating transformer I06 is suitably designed the secondary voltage of the insulating transformer may be made to difler from the secondary voltage of the controltransformer 68 by substanwhereby the requisite phase difference between the current in the windings I8 and 2|. may be obtained. In Fig. 8 we have also shown the motor I" as controlled by a line contactor III'I which is in turn controlled by a relay Il8.- Thus operation will be understood by comparison with time To, by the ratio of time oil! to time on of the curve Br after this time, and by the relative speeds of rotation of the disk in the clockwise and counter-clockwise directions as indicated by the various slopes of the curve C1 after the time T6.

It will be understood by those skilled in the art that curves of Figs. 6 and '7 have been magnified in a disproportionate manner to illustrate more clearly the principle of operation of our invention. For example, the cycle time is shown relatively long in proportion to the shape of the heating and cooling curves D and E. Under practical conditions the furnace temperature will not rise or fall as quickly as has been shown in relation to the normal cycle time, so that more cycling operations than have been illustrated will ordinarily take place during a return to normal temperature from a deviation of the magnitudes which have been shown by way of example.

By actual test it has been found that, while the total cycle time varies slightly with theloari, it remains substantially constant for all practical purposes. In an apparatus which we have'built and tested for controlling an electric furnace a total cycle time of approximately thirty seconds has been found suitable. However, it will be understood that the cycle time must be selected in accordance with the circumstances of each installation. For example, the thermal capacity of the heater, the insulation between the heated space and the surrounding space, and the time lag between energization of the heater and de livery of heat to the space will all have an effect upon the cycle time selected.

Referring now toFig. 8, wehave shown our invention in slightly different form applied to the control of an oil burner motor 65 of the type commonly used in connection with domestic heating systems. The induction relay Bi and the connected control circuit are similar to the corresponding parts of Fig. 5 and have been assigned like reference numerals. To illustrate a possible modification in the form of our invention, however, we have shown the coil 20 as wound in the same direction as the winding G9 and energized from the secondary winding of the control transin Fig. 8 when the mercury switch 23 is in its onposition the relay III is energized through the mercury switch from the secondaryterminals of the control transformer 88, and closes its contacts I09 to energize the line'contactor I01 from the secondary terminals of the control transformer 68.

.We have shown in Fig. 9 a control panel upon which is mounted an assembly of apparatus embodying our invention as shown in Figs. 5 and 8. The induction relay 6! is shown mounted at the top of the panel above the control transformer 68, the variable resistor 11 and the relay I08.

Upon the door of the panel is mounted the voltage divider II which is provided with, a manually operable handle IIII extending through the door to enable an operator to change the temperature setting of the apparatus as desired. A scale calibrated directly in temperature may be mounted upon the outside of the panel door for cooperation with a pointerattached to the shaft of the voltage divider. For indicating purposes a milliammeter H5 may be mounted in an easily Visible position upon the panel and connected in the circuit of the temperature sensitive element as indicated at Fig. s. Thus it may be seen that our invention may be embodied in a very simple and relatively small unitary apparatus suitable for easy application to a large variety of condition changing means.

Thus far we have described our invention only as applied to on-oil control systems to, reduce.

the magnitude of temperature fluctuations due to overheating and undercooling. It will be evident to those skilled in the art that a continuously oscillating temperature control system such as we have described has no diiierential between its of! and on positions, and for this reason overheating and undercoolingis minimized,

As pointed out at the beginning of this spec ification our invention may also be applied to control systems of the floating type. Such. an

arrangement is diagrammatically illustrated Fig. 10, wherein we have shown the line contactor III'I- of Fig. 8 provided with a set of normally open and a set of normally closed contacts 126 and HI respectively. The contactor to? may beconnected for energization to a control circuit similar to that shown in Fig. 8. The contacts I2Il.and I2I are arranged to control direction of rotation of a substantially constant speed reversible motor I22 which may be any oi a numberof types well known to those skilled in the art. The motor 522 is shown connected through a worm and worm gear I23 and I25 re spectively to rotate a threaded shaft I25 which From the detailed description of our control system previously given with, reference to Figs. 5, 6-and 7, it will be understood that the ratio of the time that the contacts I20 are closed to the time that the contacts I2I are closed will depend upon the relative speeds of rotation 01 the induction disk 25 in opposite directions. The speeds of rotation of the disk depend upon the magnitude of the deviation of the space temperature from its desired value. Thus, under normal conditions, the contacts I20 and I2I will be closed for equal periods of time per cycle and the motor I22 will rotate in each direction for the same length of time. Since it is assumed that the motor I22 is a substantially constant speed motor, the valve I21 will then oscillate equal distances on each side of a mean position. It now, as,due to a load change upon the system, the temperature deviates from its normal value, the time of. rotation of the motor I22 in one direction will be greater than its time of rotation in the other direction so that the mean position of the valve I21 will progress in the desired direction to allow more or less steam to pass through the conduit I28. Since the relative values of the speed of rotation of the induction disk 25 in time on to time of! will be 1:1

regardless of, the loadv on the system.

It will now be clear that we have ,described a control system which may be readily applied to almost any common on-off type of condition changing arrangement with the result that such as, for example, cooling systems, humidiopposite directions, and therefore the .relation of time oilto time on of the contactor I01, is proportional to ,the magnitude of the deviation of the temperature from its normal value, it will be apparent that the rate of progression of the mean position of the valve I21 will be proportional to the magnitude of the tempera ture deviation from normal. As the space temperature gradually returns to normal due to the greater average opening of the valve I2'I the rate of progression of the mean position of the valve will fall to zero, and the valve will take up a new mean position. When the'valve has taken up its new mean position the space temperature will of course have returned to its normal value, so that normal cycling of the apparatus will now cause the valve to oscillate equal distances to each side of the new means position. It will be understood that a reduction of load upon the system, as due to an increased outside temperature, will cause the mean position of the valve to progress in the opposite direction and cause to rest nearer to the fully closed position. It will now'be observed that in a system of this nature the mean position of the valve I2? has no fixed relation to the temperature of the temperature sensitive element 15. For this reason ii the heat load upon the system is changed so that a different value of heat input is required through the conduit I28, the valve I21 will alwaystake up a new mean position in which the heat input through the conduit I28 is just sufficient to balance the heat losses and maintain the conditioned space at precisely the desired temperature. The valve will always thereafter continue to oscillate equaldistances back and forth on either side of its new mean position. Since, therefore, there is no fixed relation between the mean position of the valve I21 and the temperature of the element 15, a system of the type illustrated in Fig. I0 does not possess the proportional characteristic which produces the undesirable subcalibration well known to those skilled in the art. In connection withthe system shown in Fig. 10 it is worth noting that whenever the temperature is at its normal value the rates of tying systems, fluid pressure systems, and the like. By way of illustration, in Fig. 10 the conduit I28 might be connected to a source of fluid pressure and to an enclosure, the pressure of which is to be maintained constant.

While we have shown and described only certain preferred embodiments of our invention, many variations and modifications will undoubtedly occur to those skilled in the art. For example, our oscillating induction relay depends for its operation upon the relative magnitude of two opposing torques, one of which is variable in accordance with the value of a condition. Each of the torques in turn has a magnitude dependent upon both the magnitude and phase relation of a certain flux to a constant exciting flux. In all embodiments of our invention which have been illustrated and described the vari-- able torque has been controlled by changing the magnitude of the flux combining with the excit-- ing flux toproduce they-torque. It will, of course, be understood by those versed in the art that, if desired, this variable torque could be controlled by maintaining constant the. magnitude of all fluxes and varying the phase relation of one flux and the exciting flux in accordance with the value of the condition being controlled. It will therefore be understood that we intend in the appended claims to cover all such modifications as fall within the-true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States, is:

l. In a system for controlling 'a condition,

movable, control means having a condition increasing position and a condition decreasing position, means for cyclicly moving said control means between said positions, and means responbetween said positions, and means responsive tosaid condition for varying inversely the speeds of said control means in opposite directions.

3. In a condition control system, condition changing means, means for controlling said condition changing means to maintain a normal value of said condition comprising a rotatable element having a condition increasing position and a condition decreasing position, oscillating means for continuously moving said element bedeviation of said condition tween said positions, condition responsive means, and means under the control of said condition responsive means to vary inversely the speeds of rotation of said rotatable element in opposite directions in proportion to the magnitude of from said normal value.

4. In a condition control system, condition changing means, means for cyclicly energizing and deenergizing said condition changing means comprising a rotatable element having spaced on and off positions, automatic reversing means operatively associated with said rotatable element for causing it to oscillate continuously between said positions and thereby maintain a predetermined normal average output per cycle of said condition changing means, said normal average output per cycle being sufficient to maintain said condition at a predetermined normal value, and condition responsive means arranged to produce an' inverse variation in the speeds of said rotatable member in opposite directions proportional to the magnitude of deviation oi. said condition from said normal value, whereby the average output per cycle of said condition changing means is regulated in accordance with said magnitude of deviation.

5. In a system for regulating the temperature of an enclosure, heating means for supplying heat to said enclosure, induction means for controlling said heating means comprising a movable element having spaced heat increasing and heat decreasing positions, oscillating means for continuously moving said element between said positions, said oscillating means including automatic reversing means adapted to be energized when said element takes up one of said positions and to be deenergized when said element takes up the other of said positions, and means responsive to the temperature of said enclosure ment between said positions, said oscillating means including automatic reversing means adapted to be energized when said element takes up one of saidpositions and to be deenergized when said element takes up the other of said positions, and means responsive to the temperature of said enclosure arranged to control the relative rates of speed of said rotatable element in opposite directions.

'7. In a system for regulating the temperature of an enclosure, heating means for supplying heat to said enclosure, operating means arranged to vary substantially instantaneously. the output of said heating means between two predetermined fixed limits, and induction relay for controlling said operating means comprising a rotatable element having spaced heat increasing and heat decreasing positions, oscillating means for continuously rotating said element between said positions, said oscillating means including automatic reversing means adapted to be energized when said element takes up one of said positions and to be deenergized when said element takes up the other of said positions, and means responsive to the temperature of said enclosure arranged to control the relative rates of speed of said rotatable element in opposite directions. i

8. In a system for controlling the temperature of an enclosure, heating means for supplying heat to said enclosure, an induction relay comprising a rotatable induction disk having spaced heat increasing and heat decreasing positions for controlling said heating means to maintain said temperature at a predetermined normal value, means for producing continuous oscillation of said elementbetween said positions cyclicly to increase and decrease the output of said heating means and thereby maintain a predetermined normal average output per cycle of said heating means, said oscillating means comprising selfreversing means controlled by the position of said rotatable element, and means responsive to the. temperature of said enclosure arranged to vary inversely the speeds of rotation of said rotatable element in opposite directions in proportion to the magnitude of deviation of said temperature from a normal value.

9. In a system for maintaining the temperature of a furnace substantially constant at a predetermined normal value, heating means Ior said furnace, relay means for controlling said heating means comprising a rotatable induction disk having spaced heat increasing and heat decreasing positions, means for applying to said disk a torque tending to rotate the disk toward one of said positions, automatic means under the control of said relay means for cyclicly changing the value of said torque between predetermined fixed limits, means for applying to said disk an oppositely directed torque having a normal value intermediate said predetermined fixed limits, and means responsive to the temperature of said furnaee for varying the magnitude of said oppo sitely directed torque in proportion to said temperature, whereby the speeds of rotation of said disk in opposite directions are varied inversely in proportion to the deviation of said tempera ture from said normal value.

10. In a system for maintaining the temperature of a furnace substantially constant at a predetermined normal value, heating means for said furnace, operating means for varying substantially instantaneously the output of said furnace between two predetermined fixed limits, relay means for controlling said operating means comprising a rotatable induction disk having spaced heat increasing and heat decreasing positions, means for applying to said disk a torque tending to rotate it toward said heat increasing position, automatic means under the control of said relay means for cyclicly changing the value of said torque between second predetermined fixed limits, means for applying to said disk an oppositely directed torque having a normal value interme diate said second fixed limits, and means responsive to the temperature of said furnace for varying the magn tude of said oppositely directed torque in proportion to said temperature, whereby the speeds of rotation of said disk in opposite directions are varied inversely in proportion to the deviation of said temperature from said normal value.

11. In a system for maintaining the temperature of a space substantially constant at a predetermined normal value, heating means for said space, a floating operator for controlling the flow of heat to said space, constant speed reversible motor means operatively connected to said floating operator, means for controlling said motor means comprising an induction relay provided with a rotatable disk having spaced on and OH positions, means for applying to said disk a torque tending to rotate the disk toward said on position, automatic means under the control of said relay for cyclicly changing the value of said torque between predetermined fixed limits, means for applying to said disk an oppositely directed torque having a normal valueintermediate said fixed limits, and means responsive to the temperature of said space for varying the magnitude of said oppositely directed torque in proportion to said temperature, whereby the speeds of rota-- tion of said disk in opposite directions are varied inversely in proportion to the deviation of said temperature from said normal value. I

12. In a system for controlling a condition, movable control means having a condition increasing position and a condition decreasing position, means for applying to said control means two normally unequal opposing forces, means operatively associated with said control means cyclicly to cause alternate predominance of said forces, and means responsive to said condition to control the magnitude of one of said forces.

13. In a system for controlling a condition, movable control means having a condition increasing position and a condition decreasing position, means for applying to said control means two normally unequal opposing forces tending to move said means in opposite directions, means operated by said control means cyclicly to vary the magnitude of one of said forces for causing said control means normally to oscillate between said positions, and means responsive to said condition for controlling the magnitude of the other of said opposing forces.

14. In a system for maintaining a condition substantially constant at a predetermined normal value, condition changing means,-control means forsaid condition changing means com prising a rotatable induction element having condition increasing and condition decreasing positions, electromagnetic means for applying to said element two normally unequal opposing torques tending to rotate said element in'oppositedirections, automatic reversing means operated by said element cyclicly to vary the magnitude of one of said torques between two predetermined fixed limits for causing alternate predominance of'said forces, condition responsive means, and electro-magnetic means controlled by said condition responsive means for varying the magnitude of the other of said forces in the value of said condition.

, 15. In a condition control system, condition changing means, an induction relay for control-- ling the output of said condition changing means comprising a movable element having spaced condition increasing and condition decreasing,

positions, an exciting winding for said induction relay, means for supplying a constant exciting fcurrent to said exciting winding, a first energizproportion to ing winding; means controlled by said movable element for cyclicly energizing and disabling said reversing winding to cause alternate predominance of said torques, and condition responsive means for controlling the energization of said second energizing winding.

16. In a system tor controlling the temperature of an enclosure, temperature changing means, an induction relay for controlling the output of said temperature changing means comprising a rotatable element having spaced temperature increasing and temperature decreasing positions, an exciting winding for said induction relay, means for constantly energizing said exciting windingto set up an exciting flux in said relay, a first energizing winding for said relay, means for constantly energizing said first energizing winding to-set up a flux differing in phase from said exciting fiux by substantially ninety electrical degrees in one sense, a second energizing winding for said relay, means for energizing said second energizing winding to set up a flux diifering in phase from said exciting flux by substantially ninety degrees in an opposite sense,

automatic reversing means for said movable element comprising a reversing winding the energization of which is controlled by said movable element cyclicly to set up a flux opposing the flux of said first energizing winding, and means responsive to the temperature of said spacefor controlling the energization of said second energizing winding.

17. In a system for controlling the temperature of an enclosure, temperature changing means, an induction relay for controlling the output of said temperature changing means comprising a rotatable element having spaced temperature increasing and temperature decreasing positions, a highly inductive exciting winding for said induction relay, 8. source of voltage .for energizing said exciting winding to set up an exciting flux in said relay, a first relatively non-inductive energizing winding for said relay, a relatively non-inductive circuit for energizing said first energizing winding from said source of voltage to set up a flux which leads said exciting flux by substantially ninety electrical degrees, a second relatively non-inductive energizing winding for said relay, means for energizing said second energizing winding from; said source of voltage to set up a flux which lags said exciting flux by substantially ninety electrical degrees, a noninductive reversing winding for said relay, means controlled by said movable element for cyclicly energizing said reversing winding to set up a flux opposing the fiux of said first energizing winding, and variable impedance means responsive to the temperature of said enclosure for controlling the energization of said second energizing winding.

18. In a system for controlling the temperature of an enclosure, temperature changing means, an induction relay for controlling'the output of said temperature changing means comprising a rotatable element operatively connected to a contact mechanism having spaced on and off positions, a 'highly inductive exciting winding for said induction relay, a source of voltage for energizing said exciting winding to set up an exciting flux in said relay, a first relatively noninductlve energizing winding for said relay, a relatively non-inductive circuit for energizing said first energizing winding from said source of voltage to set up a fiux which leads said exciting flux by substantially ninety electrical degrees, asecond relatively non-inductive energizing winding for said relay, said second energizing winding being reversely wound with respect to said first energizing winding, means comprising an adjustable voltage divider for energizing said second energizing winding from said source of voltage to set up a flux which lags said exciting flux by substantially ninety electrical degrees, a non-inductive reversing winding for said relay, means controlled by said contact mechanism for cyclicly energizing said reversing winding to set up a flux opposing said leading flux for causing alternate predominance of said leading and lagging fluxes, and variable means responsive to the temperature of said enclosure connected in circuit with said second energizing winding.

19. In a system for maintaining the temperature of an enclosure substantially constant at a predetermined normal value, temperature changing means, induction relay for controlling the output of said temperature changing means comprising a rotatable element operatively connected to contact means having spaced on and of! positions, a highly inductive exciting winding for said induction relay, a source of control voltage for energizing said exciting winding to set up an exciting fiux in said relay, a first relatively noninductive energizing winding for said relay, a

relatively non-inductive circuit for energizing said first energizing winding from said source of control voltage to set up a flux which leads said exciting flux by substantially ninety electrical degrees, a second relatively non-inductive energizing winding for said relay, means comprising an insulating transformer and an adjustable voltage divider for exciting said second energizing winding from said source of control voltage to set up a flux which lags said exciting flux by substantially ninety electrical degrees, a relatively non-inductive reversing winding for said relay connected in series circuit with said contact means cyclicly to set up a flux opposing the flux of said first exciting winding whereby said rotatable element normally oscillates continuously between said on and of! positions, and a resistor having a negative temperature coefiicient of re: sistance positioned in said enclosure and connected in series with said second energizing winding, whereby the speeds of rotation of said element vin opposite directions is varied inverse y in proportion to the deviation of said temperature from said normal value to control the average output per cycle of said temperature changing means. a

20. In a system for controlling the temperature of an enclosure, heating means iorsupplying heat to said enclosure, operating means for varying substantially instantaneously the output of said heating means between predetermined fixed limits, an induction relay or controlling said operating means comprising a rotatable element operatively connected to a contact mechanism having spaced on and oil positions, a highly inductive exciting winding for said induction relay, a source of'voltage for energizing said exciting winding to .set up a constant exciting flux in said relay, a first relatively non-inductive energizing winding for said relay, a relatively noninductive circuit for energizing said first energizing winding from said source of voltage to set =up a flux which leads said exciting flux substantially ninety electrical degrees, asecond relatively non-inductive energizing winding for said relay, means for energizing said second energizing winding from said source of voltage to setup a flux which lags said exciting flux by substantially ninety electrical degrees, a relatively non-inductive reversing winding for said relay, means controlled by contact mechanism for cyclicly energizing said reversing winding to set up a flux opposing said leading flux to cause alternate predominance of said leading and lagging fluxes, and variable resistance means responsive to the temperature of said enclosure for controlling the energization of said second energizing winding.

21. In a system for controlling the temperature of an enclosure, heating means for supplying heat to said enclosure, a floating operator for regulating the input of heat to said enclosure, constant speed reversible motor means operatively connected to said fioating operator, an induction relay for controlling said motor means comprising a rotatable element operatively connected to a contact mechanism having spaced on and oil positions, a highly inductive exciting winding for said induction relay, a source of voltage for energizing said exciting winding to set up a constant exciting flux in said relay, a first relatively non-inductive energizing winding for said relay, a relatively non-inductive circuit for energizing said first energizing winding from said source of voltage to set up a flux which leads said exciting fiux by substantially ninety electrical degrees, a second relatively non-inductive energizing winding for said relay, means for energizing said second energizing winding from said source of voltage to set up a flux which lags said exciting flux by substantially ninety electrical degrees, a relatively non-inductive reversing winding for said relay, means controlled by said contact mechanism for cyclicly energizing said reversing winding to set up a flux opposing said leading flux for causing alternate predominance of said leading and lagging fiuxes, and variable resistance means responsive to the temperature of said enclosure for controlling the energization of said second energizing winding.

22. The method of maintaining substantially constant a condition comprising continuously oscillating a control member between spaced condition increasing and condition decreasing positions, maintaining constant the speed of movement of said control member in each direction while said condition remains unchanged, and varying inversely the speeds of movement of said control member in opposite directions whenever said condition departs from its normal value.

23. The method of maintaining substantially constant the value of a condition comprising continuously oscillating a control member between condition increasing and condition decreasing positions, maintaining substantially constant the speed of movement of said control member in each direction while said condition remains unchaged', increasing the speed of movement of said control member toward said condition increasing position and decreasing its speed of movement toward said condition decreasing position whenever said condition rises above its normal value, and increasing the speed of movement of said control member toward said condition decreasing position and decreasing its speed of movement toward said condition increasing 

